1. Field of the Invention
The present invention relates to a thermally assisted magnetic recording method, for recording information on a magnetic recording medium assisted by heat.
2. Description of the Related Art
Existing recording mediums for constituting a storage unit such as a hard disk include a magnetic disk (magnetic recording medium). The magnetic disk has a stacked structure including a disk substrate and a recording layer provided with a predetermined magnetic structure. The increase in amount of information to be processed by a computer system is generating a demand for higher recording density from the magnetic disk.
When recording information on the magnetic disk, a recording magnetic head is placed close to the recording surface (including the recording layer) of the magnetic disk, so that the magnetic head applies to the recording layer a recording magnetic field stronger than the coercivity of the recording layer. The magnetic head is relatively moved with respect to the magnetic disk to sequentially invert the direction of the recording magnetic field applied by the magnetic head, so as to form a plurality of record marks (magnetic domains) having sequentially inverted magnetizing directions on the recording layer, along a circumferential direction of the magnetic disk or along the extension of the track. At this stage, the timing for inverting the direction of the recording magnetic field is controlled so as to form the record marks in the respective predetermined lengths. That is how predetermined signals or information is recorded on the recording layer represented by the variation in magnetizing directions.
In the technical field associated with the magnetic disk, it is known that the higher coercivity the recording layer has, the higher thermal stability the magnetic domain formed on the recording layer acquires, thus constituting a minute or extremely narrow and stable magnetic domain. Reducing a minimum attainable size of the magnetic domain stably formed on the recording layer leads to increasing the recording density of the magnetic disk.
Thus, when recording information on the magnetic disk, it is necessary to apply a recording magnetic field stronger than the coercivity of the recording layer, for properly forming a record mark. Accordingly, increasing the intensity of the recording magnetic field to be applied by the magnetic head could be an option, based on the increase in coercivity granted to the recording layer. The intensity of the recording magnetic field that the magnetic head can apply is, however, subject to a certain limitation from the viewpoint of the structure of the magnetic head as well as the power consumption.
As a solution, a thermally assisted magnetic recording method may be employed for recording information on the magnetic disk. When employing the thermally assisted magnetic recording method to record information on the magnetic disk, for example an optical head disposed close to the recording surface of the rotating magnetic disk emits a laser beam, so as to form a generally circular beam spot on the recording surface such that the beam spot moves thereon, thus locally heating the recording layer of the magnetic disk sequentially. The heated region of the recording layer where the temperature has been elevated incurs degradation in coercivity, in comparison with the surrounding regions where the temperature remains unchanged. Under such state, a magnetic head disposed close to the recording surface of the magnetic disk applies to the heated region a recording magnetic field stronger than the coercivity of the heated region of the recording layer, thus magnetizing a portion of the heated region in a predetermined direction. Such magnetization is fixed during a cooling process of the magnetized portion. By the thermally assisted magnetic recording method, a plurality of magnetic domains (record marks), each having a sequentially inverted magnetizing direction and a predetermined length according to the recorded signal, is thus formed along a track extending circumferentially of the disk. When employing the magnetic disk designed in accordance with the thermally assisted magnetic recording method, the recording magnetic field is applied to the region in the recording layer where the coercivity is degraded by heating, when recording information. Therefore, the intensity of the recording magnetic field to be applied by the magnetic head does not have to be largely increased, even when the coercivity of the recording layer under a normal temperature, i.e. for storing or reproducing the information, is set at a high level. Such thermally assisted magnetic disk is disclosed, for example in the following documents.
Patent document 1: JP-A-H06-243527
Patent document 2: JP-A-2003-157502
In the conventional thermally assisted magnetic recording method, the laser beam is emitted to form a generally circular beam spot on the recording surface such that the beam spot moves on the recording surface, thereby locally heating the recording layer of the magnetic disk sequentially. Heating thus the portion of the recording layer where information is to be recorded leads to a significant increase in temperature of the heated portion, as well as of a peripheral portion. Whereas, with the thermally assisted magnetic disk, it is preferable to grant the recording layer with a higher coercivity for increasing the recording density, and the higher coercivity of the recording layer requires more intense heating of the recording layer. With the conventional thermally assisted magnetic recording method, however, if the heating is so intense when recording information that the temperature is increased over a too extensive region on the recording layer, a cross-write effect may be incurred, for example the record mark excessively spreads over to another track adjacent to the track where the record mark is being formed, thus to erase or degrade the record mark on the adjacent track. The cross-write effect discourages the attempt of making the track finer, and is hence obviously undesirable from the viewpoint of increasing the recording density of the magnetic disk. Consequently, the conventional thermally assisted magnetic disk (thermally assisted magnetic recording medium) has a drawback against increasing the recording density.